One frontbencher warned voters about a feared influx of refugees, predicting "rapists and murderers" would now enter the country. And it was announced that the Christmas Island detention centre would be reopened at a cost of more than $1 billion, with plans to send sick refugees there for treatment.

The battle lines of the 2019 federal election appear to have been drawn.

SBS News

How did we get here?

University of Sydney immigration law specialist Mary Crock told SBS News that Australia's history of offshore processing goes back to the 1960s.

"Manus Island was actually set up to take refugees from West Papua," Professor Crock said.

"And we used offshore processing, in a sense, after the Vietnam War. The regional processing regime established right across South East Asia was predicated on an offshore processing-type idea; stopping asylum seekers where they are, processing them there, and distributing them in an orderly fashion."

It was Labor prime minister Paul Keating who first introduced the concept of mandatory detention for "unlawful arrivals" in 1992.

His immigration minister Gerry Hand said at the time, "the government is determined that a clear signal be sent that migration to Australia may not be achieved by simply arriving in this country and expecting to be allowed into the community".

The 2001 Tampa affair

Offshore processing catapulted into the national consciousness under Mr Keating's successor, Liberal prime minister John Howard.

In the lead up to the 2001 election, the Howard government blocked the Norwegian freighter MV Tampa from entering Australia after it had rescued 433 asylum seekers at sea.

Following weeks of limbo and relentless media coverage, the asylum seekers were loaded onto an Australian naval vessel and transported to Nauru for detention and processing.

An Australian navy ship sails near the the Tampa in 2001.

AAP

In response to the Tampa controversy, Mr Howard introduced the Pacific Solution policy, which saw asylum seekers who arrived by boat be detained and processed in offshore centres on Nauru and Manus Island.

Another centre was set up on Christmas Island, an Australian territory 350km from Indonesia. The government cut off the territory from the country's migration zone so that arrivals could not apply for refugee status once they landed there.

In a leadership defining moment, Mr Howard said during an election speech: "We will decide who comes to this country and the circumstances in which they come".

Then just weeks before the election, Mr Howard claimed asylum seekers aboard a boat bound for Australia had thrown their children overboard, in a presumed act to secure rescue and enter Australia. That claim was later found to be untrue, but Mr Howard maintains he was not told there was no evidence to support the claim.

Professor Crock said Mr Howard looked like losing the 2001 election but had "a perfect storm" of events he was able to capitalise on; boat arrivals and the 9/11 attacks in the US.

John Howard in September 2001.

AAP

"John Howard was very keyed into the electorate and also a very fortunate politician ... Howard won the election as he was able to frame stopping the boats as an act to keep people safe."

But the intensity of protests also increased both inside and outside detention centres amid disturbing reports of self-harm.

Rudd-Gillard-Rudd: 2007-2013

By the end of the Howard years, the number of boats arriving from Indonesia had dramatically dropped and offshore processing facilities had emptied.

Then-immigration minister Chris Evans said it was time for a more compassionate approach.

"Labor rejects the notion that dehumanising and punishing unauthorised arrivals with long-term detention is an effective or civilised response," he said.

A boat carrying asylum seekers arrives at Christmas Island on in 2012.

AAP

But in the months and years that followed, boat arrivals increased.

According to parliamentary records, whilst only 25 asylum seekers had travelled by boat to Australia to seek asylum in the 2007–08 financial year, that rose to more than 5,000 people during 2009–10.

Julia Gillard replaced Mr Rudd in 2010 and the issue became one that would define her prime ministership.

The uptick in boats saw Ms Gillard announce a "refugee swap" deal in which Malaysia would take 800 asylum seekers who arrived in Australia via boat in exchange for 4,000 refugees taken from Malaysia's own camps. But it was struck down by the high court.

With thousands more asylum seekers arriving and under intense political pressure, Ms Gillard announced the government would resume offshore processing in 2012.

Julia Gillard and Kevin Rudd in 2010.

Recalling those moments years later, Ms Gillard said: "We did not want people taking that journey and running those risks".

"You don't quite know what it's like as prime minister to get the telephone call ... from your defence forces that tell you that they suspect an asylum seeker boat has gone down," she said.

In 2013, Mr Rudd returned to The Lodge and announced all (rather than some) people who arrived by boat would never be resettled in Australia.

In total over the Rudd-Gillard-Rudd years, more than 50,000 people arrived and at least 1,200 people drowned at sea, according to the current government.

Coalition takes control: 2013-2019

The election of Tony Abbott to prime minister signalled an even harder line approach on boat arrivals as part of a strategy known as Operation Sovereign Borders, implemented by his immigration minister Scott Morrison and continued by successor Peter Dutton.

Key points of the Coalition's "zero tolerance" policies included the use of boat turnbacks, offshore detention and processing, a no boat arrival resettlement in Australia policy, and tight control of information around arrivals by the government.

The boat arrivals slowed and in 2015 Mr Abbot said: "In being magnificently successful we have saved the lives of hundreds of people who might otherwise have been expected to drown at sea".

His policies were continued by his successor Malcolm Turnbull and current prime minister Mr Morrison.

The US deal

In 2016, Mr Turnbull announced a new resettlement deal with the US. Former president Barack Obama had agreed to take 1,250 refugees held on Manus Island and Nauru in exchange for Central American refugees coming to Australia.

After taking office, US President Donald Trump initially criticised the swap as a "dump deal".

In a leaked transcript of his first phone call with Mr Turnbull, Mr Trump questioned whether one of the refugees would "become the Boston bomber in five years", referencing the 2013 terror attack.

A protester holds a placard during the Evacuate Manus and Nauru Protest.

AAP

Also in 2014, Iranian Hamid Khazaei died in Australia after being transferred from Manus Island. A coroner found he would still be alive if he had received proper medical care after developing a leg infection.

Images of children in prolonged detention and stories of refugee suicides were widely covered by local media.

"People become aware of the human cost and the monetary cost [of offshore detention] ... We've spent more on this than [the US$5 billion] Trump is asking for his wall," Professor Crock said.

Over several years, local refugee advocates and the international community applied pressure of the Coalition government to act.

"Any government I lead will deploy the full force of the ring of steel of the Australian defence forces and our border forces," Mr Shorten said.

Material from the Labor party says it "believes in strong borders, offshore processing, regional resettlement, and turnbacks when safe to do so because we know it saves lives at sea".

People smugglers and their customers are the only winners from Labor’s weakening of our border protection policies because ample medical treatment offshore and onshore was already available. Under Labor, it’s get on a boat, get to Nauru, get sick and get to Australia

In worrying news for Labor, a February poll taken after the Medevac bill had the party leading the Liberal-National coalition by just 51 per cent to 49 per cent on a two-party preferred, down from 54 per cent to 46 per cent in December.

It has prompted some to say that the bill could be Mr Morrison's "Tampa moment".

Professor Crock though was sceptical of such claims.

"You can never fight today's wars with yesterday's weapons ... and the problem with the fear weapon is that people can get tired of it," she said.